The inventors believe that cobalt, deposited for example by chemical vapor deposition (CVD) is attractive as a liner material for copper interconnects because cobalt provides good step coverage and is used to achieve low line and via resistance. However, the inventors have observed that, as device nodes get smaller (for example, approaching dimensions of about 22 nm or less), gap-filling processes become challenging for such cobalt liners.
The inventors further believe that ruthenium, deposited for example by chemical vapor deposition (CVD), is also a promising candidate as a liner material for copper interconnects because ruthenium advantageously improves a copper reflow process, frequently used to form a copper interconnect. A ruthenium liner also provides improved resistance to electroplating chemistry than cobalt. However, the inventors have observed that a ruthenium liner undesirably creates device performance issues, such as poor electron migration and time dependent dielectric breakdown.
Thus, the inventors have provided improved methods for forming liner layers for copper interconnect structures.